Conventionally, a wet-type friction engaging element has been used for example in an automatic transmission for motor vehicles.
As the wet-type friction engaging element of the automatic transmission, there have been used a friction plate, a brake band, etc. in a multiple disk friction engagement apparatus. As shown in FIG. 10, a multiple disk friction engagement apparatus 20 conventionally comprises a hydraulic-actuated piston 21, and a plurality of wet-type friction plates 22 and mating plates 23 arranged alternately so as to be engaged with each other by the piston 21.
The wet-type friction disk 22 comprises a core plate 24 and friction members 25, 25 fixedly attached on both sides of the core plate 24.
As shown in FIG. 11, on the surface 26 of the friction member 25 which is in contact with the mating plate 23, a plurality of grooves 30, of which sides are in parallel, are radially formed (e. g., Unexamined Patent Publication No. 53674/1997).
These grooves are for draining lubricating oil supplied for cooling, and when the wet-type friction disks with the friction members 25 fixedly attached thereon are rotated relative to the mating plates in the non-engaging state of the wet-type friction disk, the lubricating oil supplied from the center is dragged and drained towards the outer circumference.
Theoretically, torque is not transmitted between the wet-type friction disk 22 and the mating plate 23 when they are in non-engaging state. However, since there is lubricating oil between the wet-type friction disk 22 and the mating plate 23, in reality, torque is transmitted to some degree from the wet-type friction disk 22 to the mating plate 23, or from the mating plate 23 to the wet-type friction disk 22. Such torque is called a drag torque, and this may cause a loss of motive power in an automatic transmission.
This drag torque is caused by shear resistance of lubricating oil. It is known that shear resistance decreases in case lubricating oil on the friction side contains much air, but on the other hand, shear resistance increases when lubricating oil on the friction side contains little air. In addition, air content in lubricating oil on the friction surface can be increased by increasing outgoing flow of lubricating oil. Therefore, in order to reduce drag torque of the wet-type friction disk 22 and the mating plate 23, which are rotated relatively, it is effective to increase outgoing flow of lubricating oil on the friction surface, increase the content of air in lubricating oil, and thus to reduce shear resistance of lubricating oil.